I've included usable whetstone items in modules that contain a certain number of Keen Edge charges. I've been trying to think of an equivalent mundane source for a Weapon of Impact spell. Something that creates a temporary case-hardening effect perhaps?
[Edit: it seems there was an 'Oil of Impact' in 2nd edition rules which enhanced blunt weapons temporarily]
I certainly wouldn't waste a memorised spell slot on those spells, preferring to take the Improved Critical feat so that the effect is always on (for one weapon type, at least). Or you can just buy (or craft at caster level 5) a Scabbard of Keen edges. I suppose it wouldn't hurt to have a sorcerer, bard or favoured soul take the spells, even if you don't use them very often.